Method of increasing the resistance to corrosion of steel and cast iron



Sept. 25, 1934. E. H. 'scHuLz ET AL METHOD OF INCREASING THERESISTANCE 'I'O CORROSION OF STEEL AND CAST IRON Filed NOV. 29, 1930 mQg/cm biz/anions a ohul C'QYFKS Patented Sept. 25, 1934 PATENT OFFICE METHOD OF INCREASING THE RESISTANCE TO CORROSION IRON OF STEEL AND CAST Ernst Hermann Schulz and Karl Carius, Dortmund, Germany, assignors tothe firmyereinig'te Stahlwerke Aktiengesellschatt, Dusseldorf, Germany Application November 29, 1930, Serial No. 499,105

In G

Claims.

The invention relates to a method of producing steel and cast iron with increased resistance against corrosion and to the articles manufactured from said improved cast iron and steel.

The method according to the invention is an improvement in the manufacture of the composition of matter forming subject of my co-pending applications Serial Nos. 450,878 and 607,227. The composition of matter described in my former applications has a reduced tendency to rust even when attacked by water, especially sea-water and moist soil and consists of a cast iron or steel containing alloying elements which possess an electrolytic potential which is less precious than the constant oxidation potential of they cbrroding agent, but more precious than the electrolytic potential of the steel or cast iron. Due to the presence of such alloying constituent, a strongly adherent rust inhibiting coating is formed dining the corrosion process. Said protecting coating can be further improved by the presence of such alloying constituents, the ions of which by reciprocal action with the hydroxyl ions of the corrosion medium, lead to the formation of compounds of gel-like consistency. If such alloying constituents are present a firmly cemented coating without holes and fissures is formed.

The invention is based on the fact that the considerably reduced tendency for rusting, of steels with the above alloying constituents, depends on a formation of protecting layers having favourable physical properties as regards their water excluding action during the corrosion process. The process which after the accumulation of the products of corrosion on the surface of the steel due to ageing, brings a hardening and consequently .a protecting action in its train, generally that the time required for the formation of the said coatings and hence the loss of iron can be considerably reduced. If, for example, a copper steel (copper content above 0,1%) is short-circuited in a corroding medium (neutral salt solution) with a more precious metal, for example,

copper, dipping in the same solution, through a metal conductor outside the solution, asa result ermany November 30, 1929 of the tendency of the divalent iron ions passing into the solution to balance with the hydrogen ions of the corrosion medium, hydrogen ions are discharged on the more precious metal on the one hand whilst on the other hand the electro motive efiect of the oxygen dissolved in the corrosion medium leads to a depolarization of the more precious metal with the formation of water. This polarization current of oxygen which is maintained during this process flows through the exterior connecting wire from the more precious metal to the iron and, through anodic polarization, causes increased solution of the steel and its alloy constituents. The copper contained in the iron destroyed through corrosion temporarily goes into solution and at the same time is oxidized to copper ions, yet, on account of the presence of metallic iron, is immediately precipitated on the surface of the steel as a cohesive coating of metallic copper. The copper coating may be compared with a thin galvanic copper coating; it also has pores and fissures.

Through the oxygen polarization current the corrosion medium is deprived of' its oxygen to a high degree whence follows that the divalent iron ions produced in increased quantity by anodic polarization have not suflicient oxygen for complete oxidation to brown rust. Instead of this green ferro-ferrite is produced which, being gel like, adheres firmly to the surface. The ageing process of this gel consisting of ferro-fe rrite which has a hardening effect, causes the closing of the pores in the copper layer, which however is not complete since the oxygen which is supplied by diffusion from the surface of the -corrosion'medium gradually destroys the hardening ferro-fer-, rite layer through oxidation. The presence however, of such metals in the steel as alloying constituents whose ions in their highest state of valence go into solution during the corrosion process and which are capable of forming hydroxides of gel like consistency render the destructive action of the oxygen harmless. Such metals preferably are silicon, nickel and tin. Tin passes into solution as a divalent tin ion, yet through further oxidation four valent hydroxide is produced which shows favourable water excluding properties to a high degree, since, during the ageing process, it is converted into hard impermeable tin stone.

The formation of the coatings on the surface, for example, of a copper aluminum steel, by short circuiting with a more precious metal proceeds as follows:

.1. Increased passing into solution of iron and the alloying constituents contained therein.

2. Precipitation of the copper as a cohesive coating.

3. Enrichment of the voluminous tough hydroxides of the divalent iron and of the other alloying metal, for example, aluminum, which is present in addition to copper.

4. Cutting 05 of the steel surface when the hydroxides begin to age.

The hydroxides become plastic shortly after their enrichment but harden and adapt themselves to all theirregularities of the surface and completely prevent the access of the corrosion medium to the steel surface.

The coatings, that is copper, as a cohesive metallic coating and the mixture of ferro-ferrite and, for example aluminum hydroxide, have been ascertained and produced separately by analytical 'methods. By the presence of other alloying constituents such as chromium, tungsten, titanium and the like, the conditions are not substantially altered.

0n the formation of a cohesive copper coating uniformly covering the surface which is accompanied by the re-precipitation of copper rests the advantage of the method of preliminary polarization, as compared with the precipitation of copper ions in finely divided form during the corrosion of a non-polarized steel.

For thedoss of iron necessary for the formation of hydroxides capable of insuring suificient protection, is higher for cementing together the finely divided porous sponge-copper than it is for covering the continuous, coherent copper kin which has a limited number only of pores.

An essential advantage of the anodic polarization by short circuiting'the steel with a more precious metal which also results in a reduction of the loss of iron consists in that the formation of the coatings has proceeded in a very short time, about 24 hours, to such an extent that a noticeable protective action occurs, whilst without the preliminary polarization rusting steel shows a decrease in the degree of corrosion accompanied by a considerable loss of iron after 1 to 1% months only.

The current produced in the short circuited element: steel/corrosion medium/precious metal, is preferable adjusted in such a Way by choosing suitable dimensions for the metal serving as cathode, as to have a current of from 0.5 to 1.0x10 Amp/cm flowing. The development of an electromotive force, which is less than the decomposition voltage of the corrosionrnedium is naturally possible but cannot be recommended. Apart from the fact that it does not assist the process of formation of the covering layer it makes necessary the use of complicated apparatus and is costly.

In the accompanying drawing experimental results are shown graphically for the corrosion process of a copper aluminum steel with contents of 0,33% C, 0.02% Si, 0.39% Mn, 0.037% P, 0.051% S, 0.25% Cu, 0.18% Al, in artificial sea water without preliminary polarization (dashed line) and after polarization (full line). This diagram makes quite clear the improved protective eiiect of the coatings according to this process. The, abscissa shows the period of corrosion in daysand the ordinate the loss of weight in mg/cm. While the non-polarized steel requires 1 to 1% months for the formation of the coatings with a loss of weight of 0.182 gm/cm the loss after preliminary polarization with a same duration of test, amounts to only 0.07 gm/cm The ratio of the degree of corrosion of both steels is 120.38 (steel without preliminary polarization=1) that is the aluminum containing copper steel has sufiered a loss of iron which is lower by nearly The use of the new method thus ensures a considerable inhibition of corrosion of the copper aluminum steel which in itself already shows a greatly reduced tendency to rust.

The process is not limited to copper aluminum steel only but any steel which in addition to copper contains as alloying constituents, metals whose ions by reciprocal action with the hyrlroxyl ions or the corrosion medium lead to the formation of compounds of gel like consistency. separates out copper in a coherent form during the anodic polarization, due to the oxygen polarization current and the gel like corrosion products of the alloying constituents will prolong the life of the steel on account of the production of a coating which hardens and prevents access of the corrosion medium to the surface of the steel.

What We claim and desire to secure by Letters Patent of the United States is: I

l. A method for the manufacture of steel and cast iron with an increased resistance against corrosion, which consists in forming an alloy containing at least one alloying element which possesses an electrolytic potential which is less precious than the constant oxidation potential of the corroding agent, but more precious than the electrolytic potential of the steel and cast iron,

subjecting said alloy to a'corroding medium and short .circuiting it at the beginning of the corrosion process with a metal more precious than iron, thus causing the accelerated formation oi a coating from the alloying constituent.

2. A method for the manufacture or steel and cast iron with an increased resistance against corrosion which consists in forming an alloy containing at least one alloying element which possesses an electrolytic potential which is less precious than the constant oxidation potential of the corroding agent, but more precious than the electrolytic potential of the steel and cast iron, subjecting said alloy to a corrcding medium and short circuiting it at the beginning of the corrosion process with a piece of metal more precious than iron, said piece being or" such dimension as to have a current of from 0.5 to 1.0 by illamp. per square 0. in. flowing, thus causing the accelerated formation of a coating from the alloying constituent.

3. A method for the manufacture of steel and cast iron with an increased resistance against corrosion which consists in subjecting a copperiron alloy to a corroding medium and short circuiting it at the beginning of the corrosion process with a metal more precious than iron, thus causing the accelerated formation of a copper coating.

l. A method for the manufacture or steel and cast iron with an increased resistance against corrosion which consists in subjecting a copperiron alloy to a corroding medium and short circuiting it at the beginning of the corrosion process with copper, thus causing the accelerated formation of a copper coating.

5. A method for the manufacture of steel and cast iron with an increased resistance against corrosion which consists in forming an alloy containing at least one alloying element which possesses an electrolytic potential which is less precious than the constant oxidation potential of the corroding agent, but more precious than the electrolytic potential of the steel and cast iron and containing furthermore at least one al-- loying constituent, the ions of which by reciprocal action with the hydroxyl ions of the corrosion 

